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Register a new userIntense Dark Exciton Emission from Strongly Quantum Confined CsPbBr$_3$ Nanocrystals
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Dark ground state exciton in semiconductor nanocrystals has been a subject of much interest due to its long lifetime attractive for applications requiring long-lived electronic or spin states. Significant effort has been made recently to explore and access the dark exciton level in metal halide perovskite nanocrystals, which are emerging as a superior source of photons and charges compared to other existing semiconductor nanocrystals. However, the direct observation of long-lived photoluminescence from dark exciton has remained elusive in metal halide perovskite nanocrystals. Here, we report the observation of the intense emission from dark ground state exciton with 1-10 us lifetime in strongly quantum confined CsPbBr3 nanocrystals, which contrasts the behavior of weakly confined system explored so far. The study in CsPbBr3 nanocrystals with varying degree of confinement has revealed the crucial role of quantum confinement in enhancing the bright-dark exciton level splitting which is important for accessing the dark exciton. Our work demonstrates the future potential of strongly quantum-confined perovskite nanocrystals as a new platform to utilize dark excitons.
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Fully-inorganic cesium lead halide perovskite nanocrystals (NCs) have shown to exhibit outstanding optical properties such as wide spectral tunability, high quantum yield, high oscillator strength as well as blinking-free single photon emission and low spectral diffusion. Here, we report measurements of the coherent and incoherent exciton dynamics on the 100 fs to 10 ns timescale, determining dephasing and density decay rates in these NCs. The experiments are performed on CsPbBr$_{2}$Cl NCs using transient resonant three-pulse four-wave mixing (FWM) in heterodyne detection at temperatures ranging from 5 K to 50 K. We found a low-temperature exciton dephasing time of 24.5$pm$1.0 ps, inferred from the decay of the photon-echo amplitude at 5 K, corresponding to a homogeneous linewidth (FWHM) of 54$pm$5 {mu}eV. Furthermore, oscillations in the photon-echo signal on a picosecond timescale are observed and attributed to coherent coupling of the exciton to a quantized phonon mode with 3.45 meV energy.
The band-gaps of CsPbI$_3$ perovskite nanocrystals are measured by absorption spectroscopy at cryogenic temperatures. Anomalous band-gap shifts are observed in CsPbI$_3$ nanocubes and nanoplatelets, which are modeled accurately by band-gap renormalization due to lattice vibrational modes. We find that decreasing dimensionality of the CsPbI$_3$ lattice in nanoplatelets greatly reduces electron-phonon coupling, and dominant out-of-plane quantum confinement results in a homogeneously broadened absorption lineshape down to cryogenic temperatures. An absorption tail forms at low-temperatures in CsPbI$_3$ nanocubes, which we attribute to shallow defect states positioned near the valence band-edge.
2D quantum confined hybrid materials are of great interest from a solid state physics standpoint because of the rich multibody phenomena hosted, their tunability and easy synthesis allowing to create material libraries. In addition, from a technological standpoint, 2D hybrids are promising candidates for efficient, tunable, low cost materials impacting a broad range of optoelectronic devices. Different approaches and materials have therefore been investigated, with the notable example of 2D metal halide hybrid perovskites. Despite the remarkable properties of such materials, the presence of toxic elements like lead are not desirable in applications and their ionic lattices may represent a limiting factor for stability under operating conditions. Alternative, non-ionic 2D materials made of non-toxic elements are therefore desirable. In order to expand the library of possible hybrid quantum wells materials, here we consider an alternative platform based on non-toxic, self-assembled, metal-organic chalcogenides. While the optical properties have been recently explored and some unique excitonic characters highlighted, photo-generation of carriers and their transport in these lamellar inorganic/organic nanostructures, critical optoelectronic aspects, remain totally unexplored. We hereby report the first electrical investigation of the air-stable [AgSePh] 2D coordination polymer in form of nanocrystal (NC) films readily synthesized in situ and at low temperature, compatible with flexible plastic substrates. The wavelength-dependent photo-response of the NC films suggests possible use of this materials as near-UV photodetector. We therefore built a lateral photo-detector, achieving a sensitivity of 0.8 A/W at 370 nm thanks to a photoconduction mechanism, and a cutoff frequency of ~400 Hz, and validated its reliability as air-stable UV detector on flexible substrates.
For the first time the quantum dots CsPbX_3 (X=Cl, Br, I) in the fluorophosphate glasses were prepared. The samples were precipitated by two methods:(i) through self-crystallization during cooling of the glass melt and (ii) heat treatment of the glass. Controlled formation of CsPbX_3 quantum dots was realized by adjustment of cooling rate and heat-treatment conditions. The X-ray diffraction data was confirmed CsPbCl_3(Br_3, I_3) quantum dots formation. It was shown that, CsPbX_3 (X=Cl, I) quantum dots are formed in a cubic modification, while CsPbBr_3 in orthorhombic one. The photoluminescence of quantum dots have high intensity with quantum yield 45-50% and narrow band emission. The combined introduction of two anions (Cl/Br and Br/I) led to the simultaneous formation of two types of quantum dots, and indicates the difficulty of the anion exchange.
Tailoring the photoluminescence (PL) properties in two-dimensional (2D) molybdenum disulfide (MoS2) crystals using external factors is critical for its use in valleytronic, nanophotonic and optoelectronic applications. Although significant effort has been devoted towards enhancing or manipulating the excitonic emission in MoS2 monolayers, the excitonic emission in few-layers MoS2 has been largely unexplored. Here, we put forward a novel nano-heterojunction system, prepared with a non-lithographic process, to enhance and control such emission. It is based on the incorporation of few-layers MoS2 into a plasmonic silver metaphosphate glass (AgPO3) matrix. It is shown that, apart from the enhancement of the emission of both A and B excitons, the B-excitonic emission dominates the PL intensity. In particular, we observe an almost six-fold enhancement of the B exciton emission, compared to control MoS2 samples. This enhanced PL at room temperature is attributed to an enhanced exciton-plasmon coupling and it is supported by ultrafast time-resolved spectroscopy that reveals plasmon-enhanced electron transfer that takes place in Ag nanoparticles-MoS2 nanoheterojunctions. Our results provide a great avenue to tailor the emission properties of few-layers MoS2, which could find application in emerging valleytronic devices working with B excitons.
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